Imaging devices generally comprise a set of pixels each having a photosensitive semiconductor region arranged under an integrated optical filter, a color filter for example. In particular it is possible to use groups of pixels with red, green or blue filters, so as to form Bayer patterns well known to those skilled in the art.
Optical filters intended to allow only a single color through generally comprise an organic filter colored by pigments. These filters have the drawback of not being sufficiently robust, and of not being usable for infrared wavelengths.
These filters also have the drawback of being degraded when they are exposed to temperatures exceeding 200° C.
Alternatives to filters colored by pigments have therefore been proposed.
From the prior art, optical filters comprising metal layers in which features are formed with dimensions of the order of a nanometer, are known. In this respect, the reader may refer to the document “Color filter based on a subwavelength patterned metal grating” (Hong-Sik Lee, et al., Optics Express, Vol. 15, Issue 23, 2007, hereby incorporated by reference) which describes such optical filters.
Resonant optical filters for filtering one wavelength are also already known. These resonant optical filters comprise a layer of a dielectric material, generally arranged between two electrodes to form filters. The properties of the materials used to form the electrodes and the layer of dielectric material determine the resonant frequency of the resonator and therefore also the wavelength of the photons that may pass through the filter to reach a photosensitive region arranged under the filter. The resonant effect obtained in particular depends on variations in thickness and in refractive index between the electrodes and the dielectric layer. These resonators may be filters intended to let through visible colors or even infrared and ultraviolet radiation. They may in particular be arranged above imaging devices or sensors such as single photon avalanche photodiodes.
The reader may refer to document WO 2005/069376, incorporated herein by reference, which describes an imaging device comprising filters having different thicknesses for filtering various colors. The filters described in this document use materials that are not generally used for the fabrication of integrated circuits.